Process and device for melting down of scrap

ABSTRACT

A process and device for melting scrap in a direct current-driven closed shaft furnace. The scrap is fed with an annular cross-section into one end of the furnace and hot gases which flow against the falling scrap are sucked below the scrap feeding area via an outer cylindrical wall of the scrap column. The inner free surface of the ring-shaped flat cross-section of the scrap is reduced before the scrap reaches the melting zone. In the melting zone, the scrap is exposed to an electric arc at the center of the flat cross-section. The arc furnace has a cathode that is held by an electrode bearing device which is formed as a concentric pipe with respect to the central axis of the furnace vessel. At one end the pipe is conically tapered and at the other end it is supported on the shaft of the furnace by bearing arms.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention related relates to a process and an apparatus formelting down of scrap in a shaft furnace which is operated with directcurrent.

2. Description of the Prior Art

The cathodes of direct current arc furnaces are customarily supported bya support arm which is fastened on a vertically movable support column.The disadvantage of this manner of constructing electrode supportingdevices for arc furnaces such as known, for instance, from DE 37 01 678,is the large free length of electrode present with only a small heightof the shaft of the furnace vessel. The greater the structural height ofthe furnace vessel, the greater also the danger of the scrap fallingagainst the free part of the electrode resulting in the breaking of theelectrode as well as short circuits. This has the negative consequenceof an interruption in the transmission of power.

SUMMARY OF THE INVENTION

In addition, such furnaces are generally charged batchwise, so thatthere are interruptions in operation for opening the cover and removingthe electrodes. The result of this is not only a reduction in productionbut also an increase of the wear in the furnace parts, including wear ofthe refractory lining as well as of the cathode which extends into thefurnace vessel.

From German Patent 22 10 468, a process is known for the continuousproduction of steel in a shaft furnace at the bottom of which acentrally arranged electrode which extends into the furnace vessel. Theelectrode melts the charge of material arranged above it with an arc. Ashaft furnace of this construction which is intended for melting downore, pellets and sinter is scarcely suitable for melting down scrap. Onthe one hand, the necessary power cannot be introduced into the burden,while, on the other hand, the bottom electrode, which is surrounded by aslot, entails excessive risks with the level of molten bath customarywith the melting of scrap.

From EP 0 336 920, a process is known for the charging, inter alia, ofscrap in a melting or reduction-melting furnace in which the burden ispreheated before being introduced into the furnace and is held, using anelectromagnetic field, until a predetermined temperature has beenreached. In this case, the burden is subjected directly to the heat ofradiation and the process heat, namely to such an extent that, forinstance, the scrap loses its ferromagnetic properties (Curie point) andcan no longer be held by the electromagnetic field.

This melting furnace, which is designed for a continuous process, doesnot permit holding the column of scrap back in the furnace shaft.

The object of the present invention is to provide an arc furnace of thistype which, with structurally simple means, permits substantiallycontinuous operation and in which the energy necessary for melting canbe used as efficiently as possible and in which a cathode requiringlittle maintenance regardless of the height of the shaft can beemployed.

Pursuant to this object, one aspect of the present invention resides inthe process for melting down scrap in a closed shaft furnace operated bydirect current, which process includes charging the scrap into anannular chamber at a first end of the furnace so that the scrap descendsinto the furnace in a column with an annular fiat cross-section thatdefines an inner scrap-free area, drawing off heating gases which flowthrough the scrap opposite to the scrap descending direction from anouter cylindrical wall of the annular column of the scrap, reducing theinner scrape-free area before the scrap reaches a melt zone, melting thescrap down in the melt zone with an arc burning between a cathode and ananode by subjecting the scrap to the arc at a center of the flatcross-section, and tapping off melted products in a bottom region of avessel arranged at the bottom end of the shaft furnace. Another aspectof the present invention resides in an arc furnace for carrying out theinventive process, which furnace includes a shaft, of direct currentcathode that extends into the vessel, the vessel having tap holes fordischarging of the slag and/or metallic melt, an anode provided in abottom of the vessel, means for drawing off flue gas provided at a topend of the shaft, and an electrode support arranged at the top end ofthe shaft for holding the electrode. The electrode support includes apipe arranged concentrically to a center line of the shaft. Theelectrode support further has support arms connected between the shaftand a top end of the pipe. The pipe has a bottom end that is conicallytapered while the drawing off means is a flue gas duct arranged at thetop end of the shaft so as to surround the shaft.

In accordance with the present invention, it is proposed that the columnof scrap be developed in the form of a sleeve and that, in thisconnection, the scrap be allowed to descend vertically in the shaft in aflat cross section. The free inner space of the sleeve consisting ofdescending scrap is reduced before the melting zone is reached, and theinner hollow space of the sleeve of scrap is subjected to arc burningbetween a cathode and an anode.

In order to permit the completely uniform descent of the scrap, afurnace vessel is provided in the center of which the electrode isarranged within a pipe. The continuously chargeable scrap has an annularsurface in a cross-sectional plane at right angles to the center line ofthe vessel. Despite the centrally burning arc, inclined embankments arenot produced within the annular surfaces during the descent of thescrap, so that the individual cross sections remain uniformly flat.

The mounting of the electrode is provided within the pipe so that ashort free end of electrode remains and at the same time the mount is inprotected position, so that the electrode is not subjected, inparticular, to the radiant heat of the arc or of the melt. Thiselectrode end of low breakage is independent of the height of the shaft.

The top of the shaft furnace is completely free except for a covering,so that charging can be effected from any side in an unrestrictedmanner. In this way, it is possible to add charge at any desired placesof the column of scrap and thus to form a completely flat column ofscrap.

In addition to the electric melting energy, burners for gaseous and/orpowdered fuels can also advantageously be used.

The flue gases leaving the furnace are drawn off while still in theregion of the column of scrap through an annularly arranged waste-gasare fed for gas purification.

Furthermore, a device is provided for interrupting the discharge of thecolumn of scrap so that the necessary slag work can be effected withoutthe continuous sliding down of the scrap, with the result of furthermetallic melting.

In one advantageous embodiment, this stopping device is developed in theform of a magnet. This magnet is arranged both on the outer shell of theshaft and within the inner free space of the pipe. By this arrangement,a maximum magnetic holding force can be developed, the free surfacebetween the two magnets to be overcome being relatively small due to thesleeve shape of the column of scrap as compared with known structuralforms of furnaces.

In another embodiment, the stopping device consists of simple mechanicalelements which are movable in rotation or axially and reduce the crosssection of passage. These mechanical stopping elements, in the form ofbars or plates, can be introduced by simple drives into the shaft spaceof the furnace and/or are movable back and forth in the shaft itselfrelative to the wall. The mechanical elements achieve an essential partof their holding work by the clamping of individual pieces of scrapagainst each other.

In order to reduce the losses of electric energy, the feeding of thecurrent is effected though a connecting duct between the pipe wall andthe shaft wall. Due to the only slight vertical changes in theelectrode, the equalization in length of the current pipes can beobtained by a simple circular guidance. The clamping of the current feedto the electrode is effected in a protected position remote from the tipof the electrode.

The electrode is supported on lift cylinders. Since no scrap can fallagainst the electrode, there are furthermore no short circuits, with theresult that relatively slow short-stroke hydraulic units can be used inthe furnace of the invention.

One embodiment of the invention is shown in the accompanying drawing:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a section through the furnace vessel of the present usedinvention;

FIG. 2 is a top view along the line A--A of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 diagrammatically shows a shaft furnace 10 having a shaft 11 whichcan be closed by a cover 12. Flaps 16, which are opened for charging thefurnace 10 are provided on the cover 12.

In the lower vessel 13 of the shaft furnace 10 there is a tap hole 14for metal and a tap hole 15 for slag. An anode 22 is arranged in thebottom of the lower vessel 13.

In the center of the shaft furnace 10 there is an electrode supportdevice 30 which has a pipe 31 which rests, via support arms 32, on theshaft 11 of the shaft furnace 10. At its head end, the pipe 31 is closedby a cover 33. At the base end of the pipe 31, there is a conical taper34 extending towards the lower vessel 13.

Within the pipe 31 there are support elements 35 on which clamping jaws24 rest.

By these clamping jaws 24, a cathode 21 which is arranged within thepipe 31 and extends out of the base end thereof is clamped. The cathode21, which forms part of a direct-current, is supplied with electricalenergy via a lead or feed line 23. The lead 23 is in this connectionconducted over a guide roll 25.

Within a region at the mid-point of the height of the shaft 11, there isa device 40 for stopping the scrap which can be charged into the shaft.On the right-hand part of the shaft 11 the stop device 40 is constructedas an external magnet 41 and an internal magnet 42. On the left side ofthe shaft 11 the stopping device 40 is developed in the form ofmechanical means 43, 43' having rods 44 or plates 45 which can be movedby drives 46, 46'. At the to pend of the shaft furnace 10 there is aflue gas duct 19 which surrounds the shaft 11 and is in communicationwith a flue-gas purification device, not further shown.

Below the flue-gas duct 19, within the shaft 11, a connecting line 17 isprovided between the pipe 31 and the wall of the shaft 11, through whichline the energy supplies for the support elements 35, the clamping jaws24, and the current supply for the cathode 21 can extend.

FIG. 2 is a top view of the shaft 11 (section AA). In the center of theshaft 11 there is arranged the cathode 21, which is held by the clampingjaws 24. Shown in section are three support arms 32 which are fastenedat one end to the pipe 31 and at the other end to the shaft 11.

Of the stopping devices 40 proposed, only the mechanical means 43 in theform of rods 44 which are displaceable by drives 46 are shown in FIG. 2.

I claim:
 1. A process for melting down scrap in a closed shaft furnaceoperated with direct current, comprising the steps of:charging the scrapinto an annular chamber at a first end of the furnace so that the scrapdescends into the furnace in a column with an annular flat cross-sectionthat defines an inner scrap-free area; drawing off heating gases whichflow in a direction opposite to the scrap descending in the annularchamber from an outer cylindrical wall of the annular column of scrap,substantially at the first end of the furnace; reducing the innerscrap-free area before the scrap reaches a melt zone; melting the scrapdown in the melt zone with an arc burning between a cathode and ananode, the melting including subjecting the scrap to the arc at a centerof the flat cross-section; tapping-off melted products in a bottomregion of a vessel arranged at a bottom end of the shaft furnace;interrupting the descent of the scrap at a mid-point of the shaftfurnace; and removing slag from the vessel.
 2. A process according toclaim 1, wherein the melting step includes melting the scrap with aflame in the melt zone on an outer edge of the flat cross-section inaddition to melting with the arc.
 3. An arc furnace, comprising:a shaft;a vessel at a bottom end of the shaft, the vessel having tap holes fordischarging at least one of slag and metallic melt; a cathode extendinginto the vessel and operated with direct current; an anode provide in abottom of the vessel; means connected at a top end of the shaft fordrawing-off flue gas; and an electrode support arranged at the top endof the shaft for holding the cathode, the electrode support including apipe arranged concentrically to a center line of the shaft, the pipehaving a bottom end that is conically tapered, the electrode supportfurther including support arms connected between the shaft and a top endof the pipe, the drawing-off means including a flue-gas duct arranged atthe top end of the shaft so as to surround the shaft.
 4. An arc furnaceaccording to claim 3, and further comprising support elements for thecathode, the support elements being provided in the pipe so as to permita change in position of the cathode in an axial direction.
 5. An arcfurnace according to claim 4, wherein the support elements include atleast three displacement cylinders operated with a flame-proof hydraulicfluid.
 6. An arc furnace according to claim 3, and further comprisingmeans for stopping scrap from descending in the shaft, the stoppingmeans being provided in a region of a mid-height point of the shaft. 7.An arc furnace according to claim 6, wherein the stopping means includesa magnet arranged in a sectional plane of the shaft, the magnet havingat least two parts including a first part arranged to surround the shafton an outer wall of the shaft, and a second part arranged on an innerwall of the pipe.
 8. An arc furnace according to claim 6, wherein thestopping means is a mechanical means which includes rods provided to bemoveable into an interior region of the shaft.
 9. An arc furnaceaccording to claim 6, wherein the stopping means is a mechanical meanswhich includes plates provided to be moveable into an interior region ofthe shaft.
 10. An arc furnace according to claim 8, wherein the rods arefastened to the shaft in an axially moveable manner, the stopping meansfurther including drives arranged outside the shafts for moving the rodsinto and out of the shaft.
 11. An arc furnace according to claim 5,wherein the plates are fastened to the pipe so as to be pivotablymoveable, the stopping means further including drives arranged withinthe pipe for moving the plates into and out of the shaft.
 12. An arcfurnace according to claim 8, wherein the rods are fastened to one ofthe shaft and the pipe so as to be freely rotatable so that the rods canbe pressed against the column of scrap present in the shaft.
 13. An arcfurnace according to claim 9, wherein the plates are fastened to one ofthe shaft and the pipe so as to be freely rotatable so that the platescan be pressed against a column of scrap in the shaft.
 14. An arcfurnace according to claim 3, and further comprising a connectingconduit connected between the pipe and a wall of the shaft so as to forma passage, and an electric feed line arranged to pass through theconnecting conduit for providing electricity to the cathode.